Epigenetic regulation by CpG methylation splits strong from retarded IFNγ-induced IL-18BP in epithelial versus monocytic cells

Interleukin‑18 binding protein: Biological properties and roles in human and animal immune regulation (Review)

IL-18 binding protein (IL-18BP) is a natural regulatory molecule of the proinflammatory cytokine IL-18. It can regulate activity of IL-18 by high affinity binding. The present review aimed to highlight developments, characteristics and functions of IL-18BP. IL-18BP serves biological and anti-pathological roles in treating disease. In humans, it modulates progression of a number of chronic diseases, such as adult-onset Still's disease. The present review summarizes molecular structure, role of IL-18BP in disease and interaction with other proteins in important pathological processes.

Interleukin-18 and IL-18BP in inflammatory dermatological diseases

Interleukin (IL)-18, an interferon-γ inducer, belongs to the IL-1 family of pleiotropic pro-inflammatory factors, and IL-18 binding protein (IL-18BP) is a native antagonist of IL-18 in vivo, regulating its activity. Moreover, IL-18 exerts an influential function in host innate and adaptive immunity, and IL-18BP has elevated levels of interferon-γ in diverse cells, suggesting that IL-18BP is a negative feedback inhibitor of IL-18-mediated immunity. Similar to IL-1β, the IL-18 cytokine is produced as an indolent precursor that requires further processing into an active cytokine by caspase-1 and mediating downstream signaling pathways through MyD88. IL-18 has been implicated to play a role in psoriasis, atopic dermatitis, rosacea, and bullous pemphigoid in human inflammatory skin diseases. Currently, IL-18BP is less explored in treating inflammatory skin diseases, while IL-18BP is being tested in clinical trials for other diseases. Thereby, IL-18BP is a prospective therapeutic target.

Interleukin-18 Binding Protein in Immune Regulation and Autoimmune Diseases

Natural soluble antagonist and decoy receptor on the surface of the cell membrane are evolving as crucial immune system regulators as these molecules are capable of recognizing, binding, and neutralizing (so-called inhibitors) their targeted ligands. Eventually, these soluble antagonists and decoy receptors terminate signaling by prohibiting ligands from connecting to their receptors on the surface of cell membrane. Interleukin-18 binding protein (IL-18BP) participates in regulating both Th1 and Th2 cytokines. IL-18BP is a soluble neutralizing protein belonging to the immunoglobulin (Ig) superfamily as it harbors a single Ig domain. The Ig domain is essential for its binding to the IL-18 ligand and holds partial homology to the IL-1 receptor 2 (IL-1R2) known as a decoy receptor of IL-1α and IL-1β. IL-18BP was defined as a unique soluble IL-18BP that is distinct from IL-18Rα and IL-18Rβ chain. IL-18BP is encoded by a separated gene, contains 8 exons, and is located at chr.11 q13.4 within the human genome. In this review, we address the difference in the biological activity of IL-18BP isoforms, in the immunity balancing Th1 and Th2 immune response, its critical role in autoimmune diseases, as well as current clinical trials of recombinant IL-18BP (rIL-18BP) or equivalent.

Analysis of Il36a induction by C/EBPβ via a half-CRE•C/EBP element in murine macrophages in dependence of its CpG methylation level

Interleukin-36α is a novel member of the IL-1 cytokine family that is highly expressed in epithelial tissues and several myeloid-derived cell types after induction. The transcription factor (TF) C/EBPβ binds specifically to an essential half-CRE•C/EBP motif in the Il36a promoter to induce Il36a expression upon LPS stimulation. C/EBPs regulate gene expression by binding to recognition sequences that can contain 5'-cytosine-phosphate-guanine-3' dinucleotides (CpG), whose methylation can influence TF binding and gene expression. Herein we show that the half-CRE•C/EBP element in the Il36a promoter is differentially methylated in the murine RAW264.7 macrophage cell line and in primary murine macrophages. We demonstrate that C/EBPβ binding to the half-CRE•C/EBP element in the Il36a promoter following LPS stimulation is insensitive to CpG methylation and that methylation of the CpG in the half-CRE•C/EBP element does not alter LPS-induced Il36a promoter activity which correlated with similar Il36a mRNA copy numbers and pro-IL-36α protein amount in both cell types. Taken together, our data indicate that C/EBPβ binding to the half-CRE•C/EBP element and subsequent gene activation occurs independently of the CpG methylation status of the half-CRE•C/EBP motif and underlines the potential of C/EBPs to recognize methylated as well as unmethylated motifs.

Production of IL-18 Binding Protein by Radiosensitive and Radioresistant Cells in CpG-Induced Macrophage Activation Syndrome

IL-18 binding protein (IL-18BP) acts as a naturally occurring IL-18 decoy receptor. If the balance between IL-18 and IL-18BP is dysregulated, abnormal levels of free bioactive IL-18 are detected, such as in the sera of Il-18bp knockout (KO) mice with CpG-induced macrophage activation syndrome. To determine the cellular sources of Il-18bp in vivo, we selectively depleted Il-18bp expression in either radiosensitive or radioresistant cells using bone marrow transfer between wild-type (WT) and Il-18bp KO mice. Following repeated CpG injections, Il-18bp KO (donor)→ Il-18bp KO (recipient) chimeric mice exhibited more severe disease, with an enhanced Ifn-γ signature and circulating free Il-18 levels, in comparison with WT→WT chimeras. Interestingly, the phenotype of KO→WT and WT→KO mice did not differ from that of WT→WT mice. Consistent with this finding, serum Il-18bp levels were similar in these three groups of mice. The contribution of radioresistant and radiosensitive cells to Il-18bp production varied markedly according to the organ examined, with a major contribution of radiosensitive cells in the spleen as opposed to a major contribution of radioresistant cells in the lung. Finally, Ifn-γ blockade abrogated the CpG-induced but not the constitutive Il-18bp production. Our results demonstrate that circulating Il-18bp is induced in response to Ifn-γ during CpG-induced macrophage activation syndrome and is present at high levels in the circulation to prevent the deleterious systemic effects of Il-18.

IL-18/IL-18BP and IL-22/IL-22BP: Two interrelated couples with therapeutic potential

Interleukin (IL)-18 and IL-22 are key components of cytokine networks that play a decisive role in (pathological) inflammation, host defense, and tissue regeneration. Tight regulation of cytokine-driven signaling, inflammation, and immunoactivation is supposed to enable nullification of a given deleterious trigger without mediating overwhelming collateral tissue damage or even activating a cancerous face of regeneration. In fact, feedback regulation by specific cytokine opponents is regarded as a major means by which the immune system is kept in balance. Herein, we shine a light on the interplay between IL-18 and IL-22 and their opponents IL-18 binding protein (IL-18BP) and IL-22BP in order to provide integrated information on their biology, pathophysiological significance, and prospect as targets and/or instruments of therapeutic intervention.